1. Field of the Invention
The present invention relates to a wireless communication network, and in particular to a method and system for redistributing loads among the access points of a wireless communication network so as to reduce interference.
2. Description of the Related Art
The terminals of a wireless communication network generally receive control signals such as beacon signals from access points, select the strongest signal, and associate with the access point from which that signal was received. Technology of this type for connecting access points with wireless terminals is disclosed in, for example, Japanese Patent Application Publication No. 2003-249935. In the disclosed technology, communication takes place on a plurality of channels, using a carrier sensing function. A wireless terminal measures the electric field strength of the signal received from its associated access point. If the received electric field strength falls below a threshold value, the terminal may re-associate with a different access point.
In wireless networks using the above technology, an access point cannot choose the wireless terminals that will associate with it; it is the wireless terminal that decides which access point to associate with, mainly on the basis of received signal strength. This terminal-centered association strategy does not necessarily produce a well-balanced pattern of associations in the network as a whole, and can lead to unnecessary interference between wireless signals. This problem cannot be avoided by the prior art.
Interference reduces the amount of communication the network can carry, referred to as the bandwidth of the network. An example will be described with reference to FIG. 1.
The wireless network 1 in FIG. 1 is a so-called extended service set (ESS) network comprising a backbone network 3 with five access points (APs) 10-1 to 10-5 mutually interconnected by, for example, wired communication lines. Four mobile stations or wireless terminals 2-1 to 2-4 are currently present in the network area.
The fan-shaped sector depicted under each access point indicates its signal range, in which communication with wireless terminals can take place. In the areas of sector overlap, a wireless terminal can communicate with either of two adjacent access points. All wireless communications are assumed to take place on a single common channel, which simplifies the wireless connection procedure when the wireless terminals move from one access point to another. Each of access points 10-1 to 10-5 is out of the range of the adjacent access points; the access points do not communicate with each other on the common wireless channel.
In FIG. 1, two wireless terminals 2-1, 2-2 are within communication range of access point 10-2. Wireless terminal 2-1 is currently associated with access point 10-2, as indicated by the solid arrows, and is communicating with access point 10-2. Access point 10-2 and wireless terminal 2-1 form a so-called basic service set (BSS) network.
Similarly, two wireless terminals 2-2, 2-3 are within communication range of access point 10-3. Both wireless terminals 2-2, 2-3 are currently associated with access point 10-3, forming another BSS network.
Similarly, two wireless terminals 2-3, 2-4 are within communication range of access point 10-4. Wireless terminal 2-4 is currently associated with access point 10-4, forming yet another BSS network.
Under these conditions, interference may occur at access point 10-2. Access point 10-2 is communicating only with wireless terminal 2-1, but wireless terminal 2-2 is also within its signal range. If wireless terminal 2-2 begins communicating with access point 10-3, then as indicated by the dotted arrow, the signals transmitted by wireless terminal 2-2 will also be received by access point 10-2, where they may collide with signals transmitted by wireless terminal 2-1. Access point 10-2 will then be unable to receive information from wireless terminal 2-1 correctly, and will fail to transmit a reply to wireless terminal 2-1, forcing wireless terminal 2-1 to retransmit the same information. In other words, the bandwidth of the link between access point 10-2 and wireless terminal 2-1 will be reduced by an amount corresponding to the amount of traffic between wireless terminal 2-2 and access point 10-3.
For similar reasons, access point 10-4 and wireless terminal 2-4 may also find their bandwidth reduced.
Bandwidth may also be reduced on the links between access point 10-3 and wireless terminals 2-2 and 2-3.
Although no signal collisions occur at access point 10-3, which communicates with wireless terminals 2-2 and 2-3 in different time slots, signals sent from access point 10-3 to wireless terminal 2-2 may collide with signals sent from the adjacent access point 10-2 to wireless terminal 2-1, which are also received at wireless terminal 2-2. Wireless terminal 2-2 will then be unable to receive information from access point 10-3 correctly, and access point 10-3 may have to retransmit the same information to wireless terminal 2-2.
Similarly, signals transmitted from access point 10-3 to wireless terminal 2-3 may collide with interfering signals transmitted by access point 10-4, intended for wireless terminal 2-4.
In other words, the bandwidth of the links between access point 10-3 and its associated wireless terminals 2-2, 2-3 will be reduced whenever the adjacent access points 10-2, 10-4 communicate with wireless terminals 2-1 and 2-4. In the network as a whole, on the average, bandwidth is reduced in proportion to the amount of overlap between signal ranges.